Specifications of signalling systems No. 6 : recommendations Q.251 - Q.300 / International Telecommunication Union
Specifications of signalling systems R1 and R2 : recommendations Q.310 - Q.490 / International Telecommunication Union
Interworking of signalling systems : recommendations Q.601-Q.699 / International Telecommunication Union
Specifications of signalling systems No. 7 : recommendations Q.700 - Q.716 / International Telecommunication Union